Mr. Blankenhorn, the founder and president of the Institute for American Values, wrote an influential book that argued against same-sex marriage in 2007, called "The Future of Marriage," and served as an expert witness against the constitutional challenge to California's Proposition 8, which limited marriage to heterosexuals. On Friday, he said in an opinion article for The New York Times, published online, and in an hour-long documentary on NPR that his concerns about same-sex marriage remained, but that "the time for denigrating or stigmatizing same-sex relationships is over."